Radiography exposure devices which can contain sealed sources of gamma emiting isotopes, such as for example iridium-192 and cobalt-60, are extensively used in industry to nondestructively test metallic metals for defects. For example, such tests can be carried out to determine the integrity of welded connections between sections of a pipeline to be used for transporting oil. The radiography exposure device includes a shield which isolates the radiation source or gamma emiting isotopes from the operator or radiographer and the surrounding environment. A safety lock is used to removably secure the source in the shield when the device is not in use. On occasions, safety locks have failed to perform this intended function and that failure has contributed to unnecessary exposure to the radiographer and others in the immediate vicinity when the radiation source was inadvertently extended from the shield.
In the past, radiation survey meters have been used to indicate the general level of radiation about the device. However, there exists a need to provide a positive means for identifying the location and state of the source to supplement the survey meters in order to minimize the errors of the radiographers.
Position indicator lights have been used on radiographic exposure devices with generally unsatisfactory results. The switches controlling the lights occasionally malfunction, often due to the severe condition which the exposure devices are subject. As the indicator lights are the easiest available indication of source position, radiographers using the indicator lights reduce their use of accompanying survey meters owing to their belief that the indicator lights provide adequate protection. Such reliance on these indicator lights has resulted in a number of overexposures due to their malfunction. Thus again there is a need for a positive source location indicator which is not subject to the extreme conditions such as the dirty, muddy, and often gritty environment surrounding for example, the construction of an oil carry pipe.
A safety lock used in the industry includes a lock mechanism mounted at top of a knurled and rotatable cylinder. The source is mounted at the end of a flexible cable and the flexible cable is provided through a bore in the cylinder. The turning of the cylinder selectively locks the source in the shield of the radiography exposure device. Further with the shield so locked, the cylinder itself may be locked with the lock mechanism to prevent the unauthorized positioning of the cylinder.
Such a device has several problems. First, although the various positions of the cylinder are stamped thereon, it has been found that such indicia cannot provide sufficiently positive identification of the location of the source. Further, should the cylinder be returned to the locked position with the source extended from the shield, the source cannot be located in the shield until the cylinder is properly positioned.
Another prior art device includes a touch latch type lock such as, for example, found on cabinet doors wherein the latch is pushed inwardly to open and alternatively pushed inward to close. For this device to operate, first the cable on which the radiation source is mounted must be pulled out the back of the lock allowing an intermediate release of the radiation source. Concurrently with this intermediate release, an indicator provides a visual indication of the intermediate release of the source. The indicator must then be rotated in order to release the source so that it may be pushed forward, out of the shield and into position for making the necessary tests. Upon retraction of the source into the shield, the cable is pulled out the back of the safety lock, tripping the touch latch and locking the source in a safe position relative to the shield. This prior art device has the disadvantages that it is too expensive to produce and has too many moving parts which can malfunction due to mechanical fatigue or due to the introduction of dirt, grit and other contaminants into said lock.
Due to the problem of unnecessary exposure, the U.S. Nuclear Regulatory Commission published in the Federal Register, Volume 43, No. 59, on Monday, Mar. 27, 1978 an advance notice of proposed rule making set out certain guidelines for safety locks for radiography exposure devices. These guidelines are as follows:
1. The radiography exposure device shall have a lock which is not easily removable with readily available tools. PA1 2. When the device is locked, it shall not be possible to remove the source from the exposure device or to move the source shielding as to expose the source. PA1 3. The safety lock shall not prevent return of the source to a shielded position. PA1 4. It shall not be possible to unlock the exposure device with any easily available substitute for the key. PA1 5. It shall not be possible to operate the safety lock unless the source is in the fully shielded position. PA1 6. It shall not be possible to remove the source from the back of the exposure device when the safety lock is unlocked.
Further, the Nuclear Regulatory Commission has indicated that there is a need for positive indication of the position of the source. This positive position is to supplement the use of a survey meter to minimize radiographer error as to the location of the source.